Melanoma Risk in Gray Quarter Horses Studied

According to recent study results, gray Quarter Horses could be less susceptible to melanomas (malignant tumors of pigmented skin cells) compared to grays of other breeds.

Photo: The Horse Staff

Researchers have announced some good news for Quarter Horse owners and breeders: Gray Quarter Horses could be less susceptible to developing melanomas than grays of other breeds.

A University of Minnesota research team, led by Molly McCue, DVM, MS, PhD, Dipl. ACVIM, and her PhD student Raffaella Teixeira, DVM, MS, Dipl. ACVIM, recently evaluated melanoma prevalence and severity in 335 gray Quarter Horses. They hypothesized that both could be lower than in other breeds because of the predominance of the chestnut coat color (caused by a specific gene mutation) within the Quarter Horse population. Previous studies on melanoma prevalence have been limited to the Lipizzaner, Pura Raza Espanola, and Camargue breeds—all breeds in which the gray coat color is either predominant or breed-defining, Teixeira said.

The team also evaluated whether melanomas present in the study horses were affected by three genes—syntaxin-17 (STX17), which plays a role in the gray coat color; agouti-signaling protein (ASIP), which controls the distribution of black pigment; and melanocortin-1-receptor (MC1R), which plays a role in the chestnut coat color—responsible for hair coat color, two of which (STX17 and ASIP) were previously implicated in melanoma susceptibility in horses.

“Graying and melanoma have been linked to a mutation in the STX17 gene,” explained Teixeira.

The team collected data on the horses' age and sex, as well as the physical characteristics of any melanomas present. The team also assigned each horse a melanoma grade of 0-4 (0 being no melanoma and 4 being the most serious form) and collected blood or hair root samples for DNA extraction.

Key findings in the study include:

16% of the Quarter Horses had visible melanomas, a percentage much lower than that in gray Lipizzaner (50%), Camargue (31.4%), and Pura Raza Espanola (89.6%) horses.

Age was significantly associated with melanoma presence, with Quarter Horses 15 years and older more likely to have melanomas and higher grades of melanoma compared to younger horses. But again, the melanoma prevalence in older Quarter Horses (52%) was lower than that reported in Lipizzaner (75%), Camargue (68%) or Pura Raza Espanola (100%) horses.

The researchers did not find any significant associations between the melanoma presence and the STX17, MC1R, and ASIP genes.

While the researchers could not definitively tie lower melanoma prevalence and severity in gray Quarter Horses to the chestnut mutation, they said this remains a possibility. Teixeira said they are presently conducting more research “to continue to look for genetic variations that can influence melanoma risk in the horse.”

The “classification of melanoma patients into risk categories (high/low risk) based on genetic predispositions would allow veterinarians to identify patients that are candidates for early intervention,” the team said.

About the Author

Casie Bazay holds a bachelor of science degree in education from Oklahoma State University. She taught middle school for ten years, but now is a nationally certified equine acupressure practitioner and freelance writer. She has owned Quarter Horses nearly her entire life and has participated in a variety of horse events including Western and English pleasure, trail riding, and speed events. She was a competitive barrel racer for many years and hopes to pursue the sport again soon.

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